High demand for transport fuels coupled with historically high costs for petroleum has led to an increased interest in alternative methods for producing such fuels. The Fischer-Tropsch process, which can produce hydrocarbons that may be used in the production of motor fuels from CO and H2 containing syngas, is an attractive and proven method for use in such processes—provided that an inexpensive and clean source of syngas can be utilized. Fortunately in many areas, natural gas, often in the form of shale gas, is a readily available and inexpensive raw material that may be used to synthesize transport fuels. Conversion of natural gas (the primary component of which is methane or CH4) to syngas, and hence to diesel fuel, jet fuel, or gasoline via Fischer-Tropsch processes is at least conceptually simple and has been implemented for many decades. An initial step in this process is the conversion of shale/natural gas to a syngas with a specific hydrogen to carbon ratio using a reformer, typically by steam reformation. The CO-containing syngas produced in this fashion can serve as a feed stream for a Fischer-Tropsch reactor that produce hydrocarbons suitable for use as fuel.
With all such synthetic processes, however, both yield and generation of undesirable waste products are a concern. In particular, the CO2 emissions from the production of motor or hydrocarbon fuels from natural gas are a factor that makes these processes non-competitive with conventional petroleum fractionation in modern refineries.
A number of methods for capturing a portion of the CO2 produced in Fischer-Tropsch syntheses have been proposed to attempt to address this issue. Such captured CO2 may be sequestered or may be reutilized. CO2 may be extracted from the output of the Fischer-Tropsch reactor and sequestered, as disclosed in U.S. Pat. App. No. 2003/203983 and U.S. Pat. App. No. 2010/0018217; while this avoids immediate release of the CO2 to the atmosphere, establishing and monitoring such sequestration operations is not without expense. Alternatively, a portion of the CO2 may be recovered from the output of a Fischer-Tropsch reactor and subjected to a reverse water gas shift reaction in a separate reactor in order to produce a secondary CO-containing feed stream, which is directed back to the Fischer-Tropsch reactor (as described in U.S. Pat. App. No. 2003/014974), albeit at the expense of an additional processing unit. Captured CO2 may also be directed to an alternative reactor or facility to generate additional products such methanol and methanol derivatives, as described in U.S. Pat. App. No. 2004/248999, however depending on the nature of these products and how they are utilized, this may still result in a net release of CO2 to the environment.
Another source of CO2 in the production of fuel hydrocarbons from natural gas is the reformer. The primary reaction carried out in steam reformation of methane is CH4+H2O→CO+3H2; the reaction is strongly endothermic, with a standard formation enthalpy of +206 kJ/mol. Industrially, this process is generally carried out at a temperature of over 700° C. to ensure an adequate reaction rate. Since the primary reaction is endothermic, heat needs to be supplied to the reformer in order to maintain the required temperature. Typically, such reformers are equipped with burners that supply the necessary heat by utilizing a portion of the CH4 containing feed stream or of the syngas intermediate produced by the reformer itself as fuel. Unfortunately, in addition to being a significant source of CO2 emissions, this directly impacts the overall efficiency of the process by nonproductive loss of raw or intermediate materials. U.S. Pat. App. No. 2002/016375 discloses a system for producing syngas from natural gas using a reformer that increases overall efficiency by utilizing burners supplied by an external fuel source, such as a separate source of natural gas maintained for this purpose. CO2 from combustion of fuel at these burners is captured and directed to the reformer. However such CO2 capture processes are energy intensive and are unlikely to be completely effective in eliminating CO2 emissions from this source.
Alternative sources of syngas that reduce overall CO2 emissions while utilizing the economy provided by the use of fossil fuels through incorporation of biological fuel components as raw materials have been suggested. For example, U.S. Pat. App. No. 2010/0285576 describes a method that processes a slurry of coal and algae through steam hydrogasification in a gasifier to produce a CH4 rich feed stock that may be used to produce syngas, with CO2 recovered from subsequent processes utilized in cultivation of the algae. Unfortunately not only would a plant utilizing such technology would require an extensive algae cultivation facility, locations where such plants could be constructed would be limited by climate and latitude. In addition, it is unclear to what extent such recaptured CO2 would be incorporated into algal biomass.
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Unless the context dictates the contrary, all ranges set forth herein should be interpreted as being inclusive of their endpoints, and open-ended ranges should be interpreted to include commercially practical values. Similarly, all lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary.
Thus, even though various processes are known in the art to produce a feed stream for a Fischer-Tropsch process, there is still a need for improved plant configurations and processes to do so in an ecologically and economically attractive manner.